RTC: Fix compaction unit test

[alecgeatches]

What?

This PR is part of a couple related PRs for fixing a sync issue in RTC:

• Gutenberg fix: RTC: Fix divergence when two offline users reconnect #77980
• Backport to core: Backport compaction processing fix that can cause user YDocs to diverge wordpress-develop#11716

We should be able to merge the Gutenberg PR, but because core replaces the WP_HTTP_Polling_Sync_Server class, the changes in that PR won't actually be running in tests. Once the backport merges, it will, and then the test_sync_stale_compaction_succeeds_when_newer_compaction_exists() test will start failing.

Note that the fixed unit test in this PR is expected to fail until the backport is merged.

Why?

So we can fix failing unit tests once the backport PR is merged.

How?

This PR updates the unit test test_sync_stale_compaction_succeeds_when_newer_compaction_exists() to match the new server-side logic which allows multiple compaction results to be processed.

Testing Instructions

1. See CI tests pass (after Backport compaction processing fix that can cause user YDocs to diverge wordpress-develop#11716 is merged)
2. Rejoice!

Tests can also be run locally via:

 npm run test:unit:php:base -- --filter test_sync_stale_compaction_is_stored_as_update_when_newer_compaction_exists phpunit/tests/collaboration/wpHttpPollingSyncServer.php

[github-actions]

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Co-authored-by: alecgeatches <alecgeatches@git.wordpress.org>
Co-authored-by: maxschmeling <maxschmeling@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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[github-actions]

Flaky tests detected in 363decc.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/25413058813
📝 Reported issues:

• [Flaky Test] Items list is reachable via TAB #61701 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js
• [Flaky Test] can resize width #77814 in /test/e2e/specs/editor/blocks/buttons.spec.js
• [Flaky Test] post title and content are editable and blocks can be inserted #77720 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js
• [Flaky Test] should show correct homepage actions based on current homepage or posts page #77385 in /test/e2e/specs/site-editor/homepage-settings.spec.js

[ramonjd]

I just rebased this PR - if things pass I'll merge it today since the CI is blocked for others. Cheers!